Analog-to-digital-converters provide the interface between the analog world and the digital world. Analog-to-digital-converters are used, for example, in communication receivers for converting received analog signals into corresponding digital signals. When converting the analog signals, analog-to-digital-converters map analog signal values to corresponding digital signal values. Their resolution depends on the number of bits they can encode the analog signal. For example, an 8 bit analog-to-digital-converter can map the analog values to 256 different digital signal levels. However, analog-to-digital-converters are not ideal and usually suffer from at least small nonlinearities. If, for example, the analog-to-digital-converter is fed with a deterministic signal, for example, one or multiple sine waves, these nonlinearities create intermodulation products and images of the actual or wanted signals, for example at the clock frequency or multiples of the clock frequency of the analog-to-digital-converter and/or frequencies of mixed products of the clock frequency and the wanted signal. These unwanted signals are also referred to as spurious signals and superimpose the wanted signals. Especially at lower input levels, these spurious signal can be quite disturbing as they do not decrease with the input power as analog intermodulation products would.
It is state of the art to apply a dither signal to the input of the analog-to-digital-converter in a frequency range that lies outside the frequency range of the wanted signals, so that it can be suppressed by digital processing after the analog-to-digital-conversion. Dithering usually works well up to approximately −20 dBfs (fs=full scale) of the analog-to-digital-converter. If the wanted signals have higher power, the power of the dither has to be increased as well. This is also referred to as “large signal dither”. However, large signal dither can have negative side effects, for example a reduced input range for the wanted signals, as the dither signal itself adds substantial power (this may also make the automatic gain control of a receiver more difficult), and degradation of the wanted signal due to intermodulation with the dither signals.
In addition to the spurious signals, the analog-to-digital-converters typically also add noise to the signal, for example thermal noise.
Therefore, there is a need for reducing spurious signals and/or noise in devices and methods using analog-to-digital-converters.